1. Field of the Invention
This invention relates to an improvement in the tandem liquid pressure control device for an anti-look brake which is used in a motor vehicle, etc.
2. Description of the Prior Art
If the wheels of a motor vehicle are locked by receiving an excessively large braking force as compared with the force of friction occurring between the rim surfaces of the tires and the ground surface, there is every likelihood that a dangerous situation may arise, as the vehicle requires a longer distance before stopping, or it loses its directional stability or proper steerability.
Various kinds of liquid pressure control devices for anti-lock brakes have been proposed to prevent such wheel locking. This type of device mainly comprises a sensor for detecting an increase or decrease in the rotational speed of the wheels, and a control unit adapted for receiving the output of the sensor and producing an appropriate output signal to cause a reduction or rise in the pressure of a liquid which is applied to the distal ends of the brake system.
Canadian Patent No. 1,256,966 discloses a liquid pressure control device for an anti-lock brake which is constructed as shown in FIG. 7 of the accompanying drawings. This device has the advantage that the liquid pressures of the first and second liquid pressure control chambers 2 and 3 forming a tandem arrangement can be controlled simultaneously by a single differential pressure-responding member (driving member) 1. The first and second control valves 7 and 8 which are closed to cut off the supply of liquid pressure from the tandem master cylinder 4 to the front left wheel brake 5 (one of the distal ends of the brake system) and the front right wheel brake 6 (the other distal end of the brake system), respectively, are, however, operationally connected with the liquid pressure control piston 9 (first liquid pressure control piston) and the floating piston 10 (second liquid pressure control piston), respectively. The sequence of the anti-lock operation by the device is, therefore, such that it is only after the upward movement of the differential pressure-responding member 1 and the subsequent upward movement of the liquid pressure control piston 9 and the floating piston 1 that the first and second control valves 7 and 8 are closed.
This arrangement gives rise to a number of problems. If there is a great delay in the response of the differential pressure-responding member 1 which initiates the anti-lock operation, and if the tandem master cylinder 4 has a high rate of liquid supply, the first and second liquid pressure control chambers 2 and 3 and the front left and right wheel brakes 5 and 6 have a great increase in liquid pressure before the first and second control valves 7 and 8 are closed. If the seals 12, 13 and 14 slidably provided about the liquid pressure control piston 9 and the floating piston 10 along the inner wall surface of the braking liquid pressure controlling cylinder 11 are of the V-shaped type, the increase in liquid pressure of the first and second liquid pressure control chambers 2 and 3 brings about an increase in sliding resistance of the seals 12 to 14 to the extent that the liquid pressure control piston 9 and the floating piston 10 fail to follow quickly the vertical movement of the differential pressure-responding member 1. The failure of the floating piston 10 to follow the movement of the differential pressure-responding member 1 leads to a delay in the recovery of a balance of liquid pressure between the first and second liquid pressure control chambers 2 and 3 and in the opening or closing of the first and second control valves 7 and 8. This delay combines with the delay in response of the differential pressure-responding member 1 and makes it still more difficult for the device to achieve any satisfactorily quick control of the liquid pressure which is applied to the brakes.